The present invention relates to interferometers and is more particularly concerned with the measurement of angle of incidence of electromagnetic radiation incident on such an interferometer.
It is known to use a pair of antennas to detect the location of an electromagnetic signal. The pair of antennas is arranged such that the spacing between the two antennas is such that the phase difference, φ, of the signals arriving at the antennas-can be calculated using Bragg's law:
  ϕ  =            2      ⁢                          ⁢      π      ⁢                          ⁢      d      ⁢                          ⁢      sin      ⁢                          ⁢      θ        λ  where θ is the angle at which the signals approach the two antennas, d is the spacing between the antennas and λ is the wavelength of the incident radiation.
The phase difference of the incident electromagnetic signal at the two antennas can be measured and the location of the electromagnetic signal can then be determined from the angle at which the signals approach the two antennas using Bragg's law.
However, the phase difference between the signals arriving at two antennas can only be measured modulo 2π. This means that for a phase difference of φ there may be more than one value that will satisfy Bragg's law thereby producing an ambiguous result. In order to overcome this problem, it is necessary to locate the pair of antennas sufficiently close so that no matter what angle, θ, at which the incident radiation arrives at the antennas, the phase difference would never exceed 2π. In order to do this, the phase difference, φ, must be kept in the interval [−π, π), such that:
  π  >                                    2          ⁢                                          ⁢          π          ⁢                                          ⁢          d          ⁢                                          ⁢          sin          ⁢                                          ⁢          θ                λ                    .  
For incident radiation having a frequency of less than 18 GHz and an antenna specification where θ is limited such that |θ|<60°, then d=do is 9.2 mm as a suitable separation between the two antennas.
However, while this is possible in theory, it is practically impossible for the antennas to be less than a few centimeters apart. This leads to ambiguity as to how many 2π are needed to add to the measured phase to get the correct measurement.
It is therefore an object of the present invention to provide an interferometer arrangement which overcomes the problems mentioned above and provides unambiguous determinations of the angle of incidence of incident electromagnetic radiation.